The present invention relates generally to optical devices and more specifically to methods and apparatus for electronic steering of an optical beam
Precision control of optical beams is a pressing need in both the defense and commercial arenas. Precision optical beam control used for beam steering is required for laser communication, infrared countermeasures (IRCM), laser radar, and other areas. Precision beam control can also be used to implement optical switching and Nxc3x97M interconnects for free-space and fiber networks. One device for providing precision optical beam control is the optical phased array (OPA).
An optical phased array is generally a liquid crystal-based device used for beam steering, optical switching, phase control, and other optical applications. Traditional optical phased arrays (OPAs) usually only function with laser beams with a well-defined linear polarization state, whose polarization vector is parallel to the liquid crystal axis. These single polarization OPAs can be implemented with a very thin liquid crystal layer, which provides high spatial resolution with a low drive voltage.
An optical phased array device is disclosed in U.S. Pat. No. 4,639,091, issued Jan. 27, 1987 to J.-P. Huignard et al. Huignard discloses an optical phased array comprising an electro-optical liquid crystal having an upper side upon which strip electrodes are disposed and a lower side upon which a common electrode, reflective at the optical beam wavelength, is disposed. Hence, the Huignard device is a reflective device, that is, it steers a beam that is directed onto and then reflected from the device. Due to the polarization characteristics of liquid crystal, the device disclosed by Huignard works to steer optical beams only with a linear polarization parallel to that of the liquid crystal. Beams with other polarizations will be steered less effectively, or not at all.
Another optical phased array device is disclosed in U.S. Pat. No. 5,093,740, issued Mar. 3, 1992 to T. A. Dorschner. The device disclosed by Dorschner comprises a liquid crystal layer sandwiched between a layer containing a transparent common electrode and a layer containing transparent stripe electrodes. The Dorschner device is a transmissive device in that it steers the optical beam received on one side of the liquid crystal layer and transmitted from the other side. Dorschner additionally discloses the use of alignment layers in proximity with the liquid crystal molecules to properly align the molecules with the polarization of the incident light. The stripe electrodes are arranged such that the longitudinal edges of the electrodes are orthogonal to the alignment of the liquid crystal molecules. In this arrangement, an optical beam having linear polarization aligned parallel to the liquid crystal alignment will be deflected in response to control voltages applied to the electrodes, while a beam having linear polarization orthogonal to the liquid crystal alignment will pass through the liquid crystal layer undeflected. Hence, the operation of the Dorschner device is effectively limited to linearly polarized beams.
An optical phased array device for two dimensional steering is disclosed in U.S. Pat. No. 5,126,869, issued Jun. 30, 1992 to Lipchak and Dorschner. Lipchak, et al. disclose two optical phased array devices, similar to the Dorschner device discussed above, separated by a half-wave plate. The two optical phased array devices are arranged such that the alignment of the liquid crystal molecules in one device is orthogonal to the alignment of the molecules in the second device. The first optical phased array device steers a linearly polarized optical beam in one dimension. Passage of the optical beam through the half-wave plate serves to rotate the polarization of the beam 90xc2x0. The second optical phased array device, since its polarization axis is orthogonal to the axis of the first device, can then steer the rotated beam in a second dimension. Note, however, that this two-dimensional device is again limited to linearly polarized beams.
There exists a need in the art for an improved optical phased array that can provide beam control with high spatial resolution for depolarized optical beams. There also exists a need for an optical phased array for depolarized optical beams that uses low drive voltages for beam control and can be constructed from a wide range of fabrication materials. Additionally, there exists a need for such an optical phased array that can be constructed with the use of simple fabrication and packing techniques.
It is therefore a primary object of the present invention to provide an optical phased array for depolarized optical beams. It is a further object of the present invention to provide an optical phased arrays that uses low drive voltages and can be constructed from a variety of materials.
A reflective embodiment of an optical phased array according to the present invention comprises a layered structure with an optical window receiving an incident optical beam to be steered, a first transparent electrode layer, a layer of electro-optically active material, a second electrode layer, a layer of material or a device which rotates the polarization of a light beam, and a mirrored surface that reflects the light back into the material or device which again rotates the polarization of the light. Preferably, the polarization of the light beam is rotated by 45xc2x0 when it enters the material or device that rotates the polarization of the light. A voltage control device is used to control the voltages between the electrode layers, so as to create local variations in the index of refraction within the layer of electro-optically active material. The electro-optically active material is configured so as to phase shift only one linearly polarized component of the incident light beam. Typically, a liquid crystal layer provides this capability. Rotation of the polarization of the optical beam may be accomplished by a quarter-wave waveplate or other polarization rotators known in the art.
A method for steering an optical beam according to the present invention is provided by the steps of: directing the optical beam into a layer of transparent electro-optically active material sandwiched between two electrode layers, where the layer of electro-optically active material is oriented to phase shift only one polarized component; applying drive voltages to the electrode layers; rotating the polarization state of the optical beam to produce a rotated optical beam; and directing the rotated optical beam into the same layer or a different layer of transparent electro-optically active material. Preferably, the polarization state of the optical beam is rotated by 90xc2x0 to allow for optimal control over both polarization components of the optical beam.
A transmissive embodiment of an optical phased array according to the present invention is provided by: an optical window; a first upper transparent electrode layer; a first layer of electro-optically active material; a first lower transparent electrode layer positioned such that the first layer of electro-optically active material is sandwiched between the first upper and lower electrode layers; a layer of material or a device which rotates the polarization state of an optical beam; a second upper transparent electrode layer; a second layer of electro-optically active material; a second lower transparent electrode layer positioned such that second layer of electro-optically active material is sandwiched between the second upper and lower transparent electrode layers; a first voltage controller connected to the first upper and lower transparent electrode layers to control an electric field between the first electrode layers to create local variations of refractive index in the first layer of electro-optically active material, and a second voltage controller connected to the second upper and lower electrode layers to control an electric field between the second electrode layers to create local variations of refractive index in the second layer of electro-optically active material. Preferably, the polarization of the light beam is rotated by 90xc2x0 when it enters the material or device that rotates the polarization of the light. As in the case of the reflective embodiment, the layers of electro-optically active material are configured so as to phase shift only one polarization component of the incident optical beam. Preferably, layers of aligned liquid crystal material are used. The material or device to provide a rotation in the polarization of a light beam may be a quarter-wave waveplate or other polarization rotators known in the art. Voltage control over the electrodes may also be provided by devices known in the art and a single device may be used to control both sets of electrodes.
The present invention allows for the selection of a wider selection of waveplate media, which provides for improved device performance and cost, and allows for improved optical damage threshold capability and enhanced lifetime. An optical phased array according to the present invention provides spatial resolution and uses drive voltages for depolarized optical beams similar to optical phased arrays used for controlling singularly polarized optical beams.